Disable IPv6:
IPV6 got many new exciting features but if you are not on an IPV6 network, I don't think there is any benefit to having this protocol enabled.
Check here - How to disable ipv6 in Ubuntu Linux

Aliasing hostname to localhost
By putting proper entries of your localhost in your host file does improve the startup of some apps.sudo vi /etc/hosts and add you hostname to the first line after localhost like this:

127.0.0.1 localhost yourhost
127.0.1.1 yourhost

Preload:
Preload is considered to be read-ahead daemon application as it analyzes the applications which user runs, and on the basis of this analysis it predicts which applications user can run next time and launches these applications in RAM. Its daemon keeps running in background all the time and saves the required information about the applications you are running so that it may load them quickly next time.
Check here to configure preload - Speed Up Applications Load Time in Ubuntu - Preload

Swappiness:
The swappiness parameter controls the tendency of the kernel to move processes out of physical memory and onto the swap disk. Because disks are much slower than RAM, this can lead to slower response times for system and applications if processes are too aggressively moved out of memory
Check here - Performance tuning Linux Kernel with swappiness parameter

Grub Profiling:
When you boot up your machine Grub does a search for all the necessary drivers to load. This takes time. Instead of making Grub search for these drivers, the profiling actually makes Grub remember every driver necessary to work, thereby cutting down all of the driver load times.
Check here - How to speed up the booting process for Grub2 boot loader using profiling

Stop/Disable un-wanted services:
Many services are required to run all the time however many can be safely turned off for both security reasons as running unnecessary services opens more doors into your computer, but also for performance reasons
Check here - How to Start/Stop & Disable Services in Ubuntu Linux

Filesystem Access time:
Linux records information about when files was last accessed. There is a cost associated with recording the last access time. The ext3/ext4 file system of Linux has an attribute that allows the super-user to mark individual filesystem to ignore recording of last access time. This may lead to significant performance improvements on often accessed frequently changing files such as the contents of the web server directory.
Check here - Improving filesystem read performance using "noatime"

hdparm:
hdparm is a command line utility for the Linux operating systems to set and view SATA and IDE hard disk hardware parameters. It can set parameters such as drive caches, sleep mode, power management, acoustic management, and DMA settings.
Check here - Speeding up Linux Using hdparm - Optimize Hard Disk

Kernel Modules:
A Kernel Module is a program that is not actually compiled into the Linux kernel but is loaded separately. Kernel modules enable you to extend the capabilities of the kernel without the need to build extra functionality directly into the kernel image. A good example is the device driver.

Many time it happens that there are lots of module gets loaded into memory and which are not required and hence its better if we disable loading of such module to enhance the performance of the system.
Check here - How to disable loading of unnecessary kernel modules

Tune TCP/IP setting using sysctl
Tunable TCP settings can be found on /proc/sys/net/ipv4 . Here are the details of few of them.
Check here - Tune TCP/IP setting using sysctl